Tungsten-copper (W-Cu) pseudoalloys have been used as electrical contact materials and electrodes. The basic methods for the fabrication of W-Cu pseudoalloys include: infiltration of a porous tungsten skeleton with liquid copper, hot pressing of blends of tungsten and copper powders, and various techniques incorporating liquid phase sintering, repressing, explosive pressing, and the like. It is desirable to be able to manufacture articles made from W-Cu pseudoalloys at or near the theoretical density of the pseudoalloy. Besides having improved mechanical properties, the higher density pseudoalloys have higher thermal conductivities which are critical for the application of W-Cu pseudoalloys as heat sink materials for the electronics industry.
One method for producing high density W-Cu pseudoalloys consists of liquid-phase sintering of ultrafine W-Cu composite powders. Such composite powders may be produced, for example, by hydrogen co-reduction of tungsten and copper oxide blends. Another method is the direct reduction of copper tungstates. It has been demonstrated that the direct hydrogen reduction of copper tungstates imparts a high degree of phase dispersion and homogeneity to the W-Cu pseudoalloys resulting in superior thermomechanical properties. The reason for this is because copper tungstates provide a metallurgical environment were copper and tungsten are mixed together at an atomic level.
There are a number of W-Cu composite oxides in the Cu-W-O system including copper tungsten bronzes (nonstoichiometric W-Cu composite oxides of the form Cu.sub.x WO.sub.3, x=0.26, 0.34, and 0.77), cupric tungstate (CuWO.sub.4), cuprous tungstate (Cu.sub.2 WO.sub.4), and copper orthotungstate (Cu.sub.3 WO.sub.4). The copper content in these composite oxides spans the 10 to 50 wt. % copper range in the W-Cu pseudoalloys of particular interest to the industry. Most of the work to this point has focused on the reduction of cupric tungstate (CuWO.sub.4) to form W-Cu pseudoalloys. This is apparently because the relative copper content of cupric tungstate (i.e. relative to tungsten), 25.7 wt. %, is approximately in the middle of the technically important range. Adjustment of the relative copper content of the composite oxide in the 10-25% range can be accomplished by adding WO.sub.3 to CuWO.sub.4.
One technique for forming CuWO.sub.4 involves the liquid precipitation of a hydrated tungstate from aqueous solutions of CuSO.sub.4 .multidot.5H.sub.2 O and Na.sub.2 WO.sub.4 or H and CuCO.sub.3 .multidot.Cu(OH).sub.2. However, it has been found that the W-Cu pseudoalloy powders obtained from the reduction of precipitated tungstates are difficult to compact and thus the densities of the sintered pseudoalloys were low. Additionally, the solution-precipitation process is lengthy and the hydrometallurgical parameters are difficult to control.
Another technique involves the solid-phase synthesis of CuWO.sub.4 by firing intimate mixtures of equimolar proportions of CuO and WO.sub.3 (cupric tungstate may also be written as CuO.multidot.WO.sub.3). The W-Cu pseudoalloy powders obtained by the reduction of cupric tungstate made by this technique exhibit a uniform distribution of phases and desirable compacting and sintering properties. However, it is necessary to mill the CuO/WO.sub.3 mixture to form an intimate mixture prior to firing in order to reduce the diffusion distances and enhance the solid state reactions. Such intimate mixing is achieved by dry or wet milling of the reactants with hard metal or ceramic milling media. Unfortunately, the energy of mechanical impact and friction inevitably cause erosion of the milling media and contamination of the material being milled. Wet milling can additionally promote undesirable chemical reactions between the reactants and the milling media thereby adding to the contamination of the mixture. Contamination of the reactant mixture from the milling process can seriously impair the use of W-Cu pseudoalloys as heat sink materials. Thus, since the milling step increases the risk of contamination and adds another step to the manufacturing process, it would be desirable to have a method of producing W-Cu composite oxide powders having similar characteristics without having to mill the reactants to form an intimate mixture.